1. Field of the Invention
The present invention relates to liquid phase epitaxial (LPE) growth of compound semiconductors, more particularly, to an LPE growth method of a compound semiconductor layer in which a supercooling condition can be generated without controlled cooling.
2. Description of the Prior Art
Light emitting devices, for generating light medium information signals and light receiving devices for detecting the light, are used in various fields, such as optical communications, industrial equipment, and civilian life. Considerable research and development has been carried out on such devices.
Optical communications make use of AlGaAs/GaAs double hetero junction lasers, silicon PIN photo diodes, and silicon avalanche photo diodes, which are suitable for light having a wavelength of about 0.8 .mu.m. The recent success in decreasing the transmission loss of optical fibers has also led to the development of semiconductor light emitting and light receiving devices suitable for light having a wavelength of from 1.0 to 1.7 .mu.m.
Information terminal units or devices used in civilian life make use of semiconductor light emitting and light receiving devices for light having a shorter wavelength, by reasons of visibility, range of usable sensitized materials, flexibility of optical system design, safety codes, etc.
In order to produce semiconductor light emitting and light receiving devices having a desired wavelength character, an active layer of the devices is made of a semiconductor crystal with the band gap corresponding to the desired wavelength. The active layer and clad layers lying on both sides of the active layer are formed on a semiconductor single crystalline substrate by epitaxial growth. Between the active layer and the clad layer and between the clad layer and the substrate a hetero junction is formed. The hetero junction should have a smaller lattice distortion by matching the lattice constants of the active layer, the clad layers, and the substrate, to each other.
Epitaxial growth methods include LPE, vapor phase epitaxy (VPE), chemical vapor deposition (CVD), molecular beam epitaxy (MBE), and organic metal chemical vapor deposition (MO-CVD). The most suitable method is selected depending upon the conditions of the desired grown layer, such as the composition or thickness. The LPE method has been most extensively used because of its superiority. Since the properties and reliability of the semiconductor light emitting and light receiving devices depend on the crystal of the layer grown by the above-mentioned method, further improvements for elimination of crystal defects and stabilization of crystal composition are required.
The conventional LPE growth method of compound semiconductors includes a temperature drop method and temperature difference method.
In the temperature drop LPE method, the temperature of the semiconductor material solution is reduced over a certain period to generate a supercooling condition in the solution, whereby a crystal is epitaxially deposited on the semiconductor substrate to grow an epitaxial layer. However, the supercooling degree necessary for growing good crystal sometimes cannot be attained. Also, it is difficult to suppress composition variation of the growing layer and to increase the uniformity of the growing layer composition.
In the temperature difference LPE method, a temperature difference (i.e., a temperature gradient) is provided in a semiconductor material solution in the vertical direction to form a concentration gradient of the solute in the solution, whereby a crystal is epitaxially deposited on the semiconductor substrate to grow an epitaxial layer. However, it is not easy to carry out this method in practice, because the uniformity of the substrate surface temperature should be maintained and, at the same time, as large a temperature difference as possible should be provided in the vertical direction.
Below, a discussion of the growth of an indium-gallium-phosphide (In.sub.1-x Ga.sub.x P) layer on a gallium-arsenide (GaAs) substrate with a (100) face by the temperature drop LPE method, is presented.
Weighed semiconductor materials (In, InP, and GaP) with a desired weight ratio are put in a reservoir (i.e., a through hole) formed in a slider of a conventional slide type graphite boat. The materials are heated at an elevated temperature, e.g., 800.degree. C., to melt them into a solution. In order to obtain a supercooling condition of the solution, the solution is cooled to a certain extent. A degree of supercooling of at least 5.degree. C. is required for epitaxial growth at a temperature of about 800.degree. C. To obtain an especially good epitaxial layer, a degree of supercooling of about 10.degree. C. is required. Accordingly, after the solution is cooled to about 10.degree. C., the slider brings the solution into contact with the GaAs substrate, whereby an InGaP crystal is precipitated on the substrate to form an InGaP layer. However, since the temperature of the solution is not uniformly decreased, namely, a temperature difference between the central part and the outer part of the solution occurs, the supercooling condition is easily broken during the cooling. Accordingly, it is very difficult to achieve a supercooling condition of at least 5.degree. C. in practice. In this case, the desired In.sub.1-x Ga.sub.x P layer cannot be grown with good reproducibility, since crystal defects, dislocations, voids, and the like are generated in the grown layer.
Furthermore, the lattice constant of the In.sub.1-x Ga.sub.x P epitaxial crystal depends strongly on its composition. To form a high quality InGaP epitaxial layer, it is necessary to keep the variation of the solution composition as small as possible during the growth period. Thus, the conventional LPE method has limits as to the thickness and area of the grown layer.